1. Field of the Invention
This invention relates to an apparatus for drying a web of paper emerging from a press section of a papermaking machine. More particularly, this invention relates to an apparatus for drying a web in which the web is transferred without open draw between dryer sections, thereby permitting threading of the web without the assistance of threading ropes and the drying of both sides of the web while continually restraining the web by felts.
2. Information Disclosure Statement
With the ever increasing operational speed of papermaking machines, a serious problem has existed in that there is a tendency for the paper web or sheet to flutter as the sheet progresses through the dryer section. Such sheet flutter is particularly evident when the sheet is transferred between succeeding sections of the dryer section as the web is transferred between these adjacent sections in an open draw. Such sheet flutter has been minimized by the use of single felting configurations in which the web and felt run jointly between respective top and bottom cylinders. However, the single felt configuration, although reducing the aforementioned problem of sheet flutter, introduces several disadvantages. Included amongst these disadvantages are, first, the heat transfer from the bottom cylinders is substantially reduced because the wet web is no longer in direct contact with the cylinders, the felt being interposed between the web and the drying surface of the respective cylinder. Second, the web has a tendency to separate from the felt as the web travels towards and around and then away from the bottom cylinder. Third, the initial threading of the web is not particularly easy.
A partial solution to the aforementioned single felt problems has been provided by the application of the so-called BelRun dryer section. BelRun is a registered trademark of Beloit Corporation. With the BelRun system, the bottom, ineffective dryers are replaced by vacuum rolls which positively convey the web from one cylinder to the next. Recent installations of this type of dryer section have shown that the BelRun concept can be extended to include a large number of dryers without any adverse effect on the web runnability. Such runnability results because the vacuum rolls are capable of conveying the web along the felt-supported spans without the need for sheet tension or section draw points.
With the implementation of the single BelRun section, there exists a tendency to have a generation of stresses which develop in the web as the web dries. Such stresses impart a tendency for the dried paper to curl. "Curl", according to "Pulp and Paper Dictionary" by John R. Lavigne, published 1986, is defined as "a paper or paper board deformation caused by non-uniform distribution of strains and stresses throughout the sheet as a result of an even internal moisture and conditioning." Such adverse curling effect can be minimized or eliminated by drying the web from both sides, but two-sided drying requires a transfer point in which the web is transferred from one felt t another felt. In the case of the present invention, the web must be alternately dried on a top tier dryer section and then on a bottom tier dryer section. A top tier section may be defined as a group of dryers in which the bottom surface of the web contacts the dryers. A bottom tier section conversely and correspondingly may be defined as a group of dryers in which the top surface of the web contacts the dryers.
In order to efficiently transfer the web from one section to another, a positive transfer arrangement is used. In the prior art, such means for transferring the web from one drying section to the next has required the introduction of an open draw with the associated problems of sheet flutter and the like.
Modern paper drying machines are contemplated in which web speeds of 10,000 or more feet per minute are envisaged. Consequently, the introduction of such open draws would lead to serious problems of sheet flutter and numerous web breakages. The present invention seeks to overcome the aforementioned inadequacies of the prior art apparatus and to provide a drying apparatus which contributes a significant and non-obvious contribution to the paper drying art.
Another object of the present invention is the provision of an apparatus for drying a web of paper emerging from a press section of a papermaking machine such that the web is transferred between a first and a second dryer section means without open draw, thereby permitting both threading of the web without the assistance of threading ropes and the drying of the web on both sides thereof.
Another object of the present invention is the provision of an apparatus for drying a web of paper which completely eliminates the need of open draws.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which open draws for the sheet or web are eliminated.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which blow boxes would be redundant.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which ventilation of the pockets defined by the cylinders and the vacuum rolls is improved, thereby improving the drying rate of the web.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which gear case leaks are inhibited and removal of broke is facilitated.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which fewer steamfits are required and none of the dryers are redundant, thereby reducing the blow through rate.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the frame is symmetrical and in which the base frame is subjected to equal loading forces, thereby resulting in a sturdy, low-profile frame which reduces vibration and its attendant noise level.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the power required to drive the cylinders is reduced and due to the layout and configuration of the dryers, such arrangement lends itself to the provision of a low-profile hood.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which runnability of the drying apparatus is increased and in which doctors can be applied to each dryer.
Another object of the present invention is the provision of an apparatus for drying a web of paper which permits automatic threading of a tail and subsequent web therethrough.
Another object of the present invention is the provision of an apparatus for drying a web in which open access to the dryers and vacuum rolls is provided and a supply of air can be fed uniformly through each of the vacuum rolls.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which hoods are provided over and adjacent the vacuum rolls for handling the humid exhaust and possibly eliminating the need for large scale exhaust hood construction.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which such vacuum roll hoods can be provided for profiling the web by removing exhaust air from selected transfer sections of these hoods.
Another object of the present invention is the provision of an apparatus for drying a web ranging from lightweight grades to heavy board.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the humidity is decreased by eliminating the closed pockets associated with a typical two-felt type drying section.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the apparatus is no longer than a conventional dryer section but requires less equipment.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which runnability is improved by eliminating open draws.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the felt-supported draw is reduced to a minimum.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which sheet control and restraint is provided by the direct application of vacuum.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which there is no need for residual sheet tension to hold the sheet against the felt since the sheet is entirely supported by vacuum as it wraps the transfer roll. In a conventional Uno-Run dryer section, a draw is required in order to induce a tension in the sheet to hold the sheet as it wraps the bottom dryer. Although blow boxes can be used to prevent the sheet from leaving the felt, the vacuum produced by these boxes is quite low--in the order of 0.05 to 0.1 inches WC (water column). Furthermore, the vacuum does not extend around the bottom dryer, and it is generally not sufficient to hold the sheet against centrifugal forces, and residual sheet tension is required to hold the sheet against the felt.
In the arrangement according to the present invention, the transfer roll vacuum is not so limited and, typically, 4 inches WC is applied to the web, which is more than four times the level needed for sheet support.
Another object of the present invention which is a less obvious factor relative to the runnability of the dryer section is the uniformity of dryer surface speeds. Such uniformity is obtained because the dryers are driven by the felt instead of by a gear train. Dryers which are geared together are forced to run at equal rotational speeds. In the case of dryers which have slightly different diameters, perhaps due to manufacturing tolerances or differences in steam pressure and temperature, such differences result in unequal dryer surface speeds. These unequal surface speeds not only increase the drive load but also cause problems with sheet runnability.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which threading of the apparatus is simplified.
More specifically, in a conventional Uno-Run system, the tail tends to separate from the felt on the bottom dryers and wander in the cross-machine direction as it moves down the machine. In the dryer section, according to the present invention, the vacuum transfer rolls are equipped with internal dampers for concentrating the vacuum in front-side threading chambers. When these dampers are closed, the vacuum in the threading chambers is increased from 4 inches WC to 10-12 inches WC. Such vacuum holds the tail tightly to the felt and prevents the tail from wandering and stabilizes the entire threading operation.
Additionally, when the apparatus, according to the present invention, is extended through the entire dryer section, the tail can be threaded without the use of ropes. Special air nozzles are located near the edges of each dryer to insure that the tail follows the felt and is transferred by the vacuum rolls. These air nozzles eliminate the need for threading ropes, threading equipment and maintenance. Furthermore, the aforementioned threading arrangement helps to insure safe operation of the apparatus.
Also, when the ropes are removed, a wider felt can be used which tends to further stabilize the edges of the sheet on the felt.
With the open symmetrical framing, according to the present invention, broke removal, dryer access and dryer visibility are greatly improved. Additionally, although not a requirement of the present invention, each of the dryers can be easily fitted with a doctor, such doctors being unloaded automatically with air cylinders if required.
The apparatus, according to the present invention, permits a significant reduction in the amount of paper machinery which is required to meet the design production. In addition to reducing the number of dryers, felt rolls and guides, there is also a corresponding reduction in the number of steamfits, the elimination of blow boxes and pocket ventilation ducts. Also, the present invention enables simplification of the dryer framing.
With the application of the "Silent Drive" dryer system (Silent Drive is a registered trademark of Beloit Corporation) and, additionally, with the low profile afforded by the framing layout of the present invention, dryer section noise and vibration will be reduced to a minimum. Furthermore, smaller drive motors can be installed due to the fact that the section inertia is reduced in view of the elimination of the bottom dryers utilized in a typical Uno-Run system. Additionally, the dryer hood can have a low profile construction, and hood door lifts can be installed on the back as well as the front of the machine.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the energy efficiency of the apparatus is improved.
More specifically, the dryer section of the present invention provides improved energy efficiency in the following three areas. First, the drive load is reduced by having fewer dryers and less steamfit drag. Second, the amount of blowthrough steam is reduced by minimizing the number of dryers. Third, the large blowthrough quantities associated with the bottom dryers of conventional Uno-Run type sections are entirely eliminated.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the quality of the sheet is improved.
More specifically, the dryer section, according to the present invention, has the capability of affecting the sheet properties as follows. First, the tensile strength and stretch of the web in the machine direction is improved. Second, the tensile strength and stretch of the web in the cross-machine direction is improved. Third, the tendency for the sheet to curl is reduced, and fourth, the tendency to generate edge cockles is reduced. According to the aforementioned "Pulp and Paper Dictionary" by John R. Lavigne, "cockle" is defined as "a paper defect appearing as a wrinkle caused by non-uniform shrinking due to uneven drying or sheet formation." Furthermore, "cockling" is defined as "the process of producing a ripple-like defect or intentionally creating this type of paper surface by controlled, uneven drying of the sheet under minimum tension conditions."
Once the sheet leaves the press section, the machine direction tensile and stretch are affected primarily by the press section draw and somewhat less by the draw between dryer sections. In conventional drying sections, these dryer draws are usually set as low as possible, but the minimum draw is dictated by runnability and sheet control. However, in the drying section, according to the present invention, there is no minimum draw requirement for runnability. Nevertheless, the machine direction properties can still be affected in the usual way by pulling a draw between sections if such is deemed desirable.
In a somewhat similar manner, the cross-machine direction tensile and stretch are affected by the cross-machine direction sheet restraint. In an open draw, there is no restraint, so the sheet freely shrinks (although this shrinkage is greatest at the edges). In operation of a typical Uno-Run type drying section, there is a degree of web restraint as evidenced by the increase in trim and the decrease in edge cockles.
In the drying section, according to the present invention, the sheet restraint is more positive than that provided by a typical Uno-Run system, and this further reduces any tendency for edge cockles to develop. With the improved restraint provided by the present invention, a slight increase in cross-machine direction tensile and a decrease in cross-machine direction stretch is evident. The decrease in cross-machine direction stretch is most pronounced at the edges where the sheet normally has the least restraint. Accordingly, the net effect, according to the present invention, will be a sheet with more uniform properties in the cross-machine direction.
The propensity to curl is caused by three factors as follows. First, fiber orientation, second, fines and filler (bonding) distribution, and, third, residual fiber stress.
The dryer section affects only the fiber stress. Curl is normally controlled by adjusting the drying from each side of the sheet by separate control of the top and bottom dryer steam pressures.
Accordingly, in the drying section, according to the present invention, such control is provided by adjusting the steam pressures in subsequent sections.
Another particularly important object of the present invention is the provision of an apparatus for drying fine paper grades where directionality is important. More particularly, restrained drying of the web is an important feature of the present invention in that it will not permit the edges of the sheet to shrink more than the center portion of the sheet. The key to restraining the sheet while it is not held between the felt and the dryer drum is the vacuum in the transfer, or turning roll, below adjacent drums and the vacuum means for transferring the web between dryer sections. The sheet may be restrained, as stated hereinbefore, by application of a vacuum within the range 1-10 inches WC and preferably approximately 4 inches WC.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the following detailed description taken in conjunction with the annexed drawings.
Although the detailed description and annexed drawings describe a preferred embodiment of the present invention, it should be appreciated by those skilled in the art that many variations and modifications of the present invention fall within the spirit and scope of the present invention as defined by the appended claims.